PROJECT SUMMARY/ABSTRACT The research described in this proposal is aimed at answering four fundamental questions that relate to the regulation of growth: (1) How is the growth of many individual cells regulated so that they collectively generate a tissue or organ of a specific shape and size? (2) How does the growth of specific tissues affect the developmental progression of the entire organism? (3) How does an organ sense injury or damage and then activate a program to regenerate missing tissue? (4) Why does the capacity for regeneration diminish as an organism matures and are there ways of improving regenerative capacity? Many human diseases such as birth defects and cancer are characterized by an abnormal regulation of growth. Understanding the mechanisms that regulate growth will therefore point to novel therapeutic strategies for these conditions. Our studies will also suggest ways for improving regeneration following tissue damage. To obtain a better understanding of both growth during development and regenerative growth, we use a genetic approach in the fruit fly Drosophila melanogaster. By screening for mutations that make cells outgrow their wild-type neighbors, we have identified many genes whose normal function is to restrict tissue growth including components of the Hippo pathway. The ortholgs of some of these genes have been shown to be tumor-suppressor genes. To study regenerative growth, we have generated a system where tissue in the imaginal disc can be ablated by a temperature shift. We have used this system to show that many pathways important for regenerative growth are activated less efficiently as the organism matures. In the case of Wnt gene expression, this is due to localized epigenetic silencing of a damage responsive enhancer. The goal in the coming years is to continue the characterization of mutations that regulate the extent of tissue growth, and to identify additional genes that regulate cell growth, cell competition and cell affinity using the CoinFLP method. To assess the activity of growth regulating pathways in individual cells, quantitative imaging techniques will be used to measure changes in nuclear localization of transcriptional regulators as the tissue approaches its final size. The role of secreted proteins that communicate the growth status of individual organs to the neurodendocrine system, so as to regulate the timing of development, will be studied. To characterize changes in regenerative capacity, we will study multiple damage-responsive enhancers in detail as well as use genome-wide approaches to characterize changes in chromatin states elicited by tissue damage. We will use a novel system, Dual Control, to screen for genetic manipulations that improve regeneration in mature tissues.